One of the grading metrics is writing. None of them is for art. It's not an ethical question. It's a "I want to see the adventure you WRITE!" question. The art is kind of irrelevant to that end. Sure art helps make the product look professional, but you don't need it. And to that end, AI images make the product look unprofessional.
Knave 2e has plenty of spreads with no art. Ben's adventure The Waking of Willowby Hall is practically artless (just nix the portraits). Art is icing on the cake that is your adventure. In contrast, you can't turn in an adventure with no writing . It's absolutely required.